We took DD in January at 20 months and I was amazed at how much there was for her to do in all the parks.
Here are the things she particularly enjoyed:
Magic Kingdom - Flying Dumbo, Carousel, Winnie the Pooh, Peter Pan, Move it Shake it parade (where they stop and dance with the crowd), It's a Small World
Animal Kingdom - Kilimanjaro Safari
Hollywood Studios - Playhouse Disney Show (now Disney Junior), Honey I Shrunk the Kids play area
Epcot - Character meets, Spaceship Earth, Donald Duck boat ride in Mexico.
She loved the atmosphere in the parks, loved the parades, the interactions with the cast members - even the buses to the parks. We took her on anything that didn't have a height restriction - even the Haunted Mansion ride, and she loved everything. She still talks about it now.
The flight was fine, much better than I expected. We took a couple of magazines (Cbeebies type), crayons and food and she was fine. In fact she played most with the tv control thingy, pretending it was a phone. Virgin also gave her a bag with a few bits in which she was delighted with.
You will have a great time, don't worry about it just enjoy it. I wish I had worried less beforehand! Hope all goes well with your DH's op.